1. Field of the Invention
The invention relates to an apparatus for surgically modifying the curvature of the eye cornea and a method of controlling the apparatus, and more particularly to an apparatus for smoothly correcting a variety of corneal defects using a large, fixed spot size in an overlapping pattern that reduces thermal effects.
2. Description of the Related Art
Since the initial development of corrective lenses, 20 new and better ways of correcting defective eyesight have been developed. From the bifocal lens and extended wear soft contact lens to corneal incisions and shaping, the field of ophthalmology has seen great advances in convenience, safety, and accuracy in correcting a variety of sight defects, including myopia, hyperopia, and astigmatism.
While corrective lenses still find wide general application, ophthalmologists are focussing on surgery to correct such defects. One of the most popular surgical techniques is radial keratotomy, in which a surgeon forms radial slits in the outer surface of the cornea, allowing the cornea to re-shape and resulting in a modified cornea to correct the deficiencies of the patient's sight. This technique has continued to develop, but the advent of the laser and its introduction into the field of medicine have given rise to a new and potentially revolutionary method of eye surgery. Specifically, the development of the excimer laser and its application to eye surgery has opened a new approach to ophthalmological surgery.
The excimer laser produces coherent light of a very short wavelength of around 193 nm. At these wavelengths and the resulting high energies, the excimer laser removes, or ablates, tissue at the molecular level without significant heating of adjacent tissue. Thus, rather than "burning" away tissue, the excimer laser literally breaks the molecular bonds, and the ablated tissue is ejected from the ablated surface leaving a relatively unmarred surface to heal virtually scar-free. This aspect of the excimer laser is now well known and is further described, for example, in U.S. Pat. No. 4,784,135 entitled "Far Ultraviolet Surgical and Dental Procedures," issued Nov. 15, 1988.
The word "excimer" in excimer laser was initially drawn from its molecular principal of operation. The excimer laser was initially based on the lasing action of excited dimers, such as xenon, krypton, or fluorine in the form of Xe.sub.2, Kr.sub.2, or F.sub.2. The word "excimer" as applied to lasers is now a misnomer, as the most popular excimer laser used in eye surgery does not even use dimers--it uses argon fluoride. The excimer laser is also a pumped laser, in the sense that another laser is used to stimulate the lasing action of the argon fluoride mixture in the laser cavity. "Excimer laser" has now come to be applied to an entire group of lasers with ultraviolet wavelengths below 400 nm.
When used in ophthalmological surgery, the excimer laser is preferably pulsed, as that allows for application of high energies without thermal heating. These pulses are very short bursts of high energy laser light applied to the cornea. For example, such a laser is typically pulsed at between 1 to 50 Hz with a 10 to 20 ns pulse duration. A drawback of the excimer laser, however, is the energy density over the beam tends to have both large and small scale inhomogeneities. The application of the excimer laser for surgical procedures is described in U.S. Pat. No. 4,784,135, entitled "Far Ultraviolet Surgical and Dental Procedures," issued Nov. 15, 1988. For a historical background of the development and application of the excimer laser to ophthalmic surgery, see Chapter 1 of the Color Atlas/Text of Excimer Laser Surgery, .COPYRGT. 1993 Igaku-Shoin Medical Publishers, Inc.
As early as 1983, researchers recognized the potential application of excimer laser light in reshaping the cornea. Since that time, a number of systems have been developed to reshape the cornea, using a variety of techniques such as variable sized circular apertures to correct for myopia, variable sized ring shaped apertures to correct for hyperopia, and variable sized slit shaped apertures to correct for astigmatism. These techniques collectively came to be known as photorefractive keratectomy. It has been recognized that using such apertures to correct for myopia, for example, a series of excimer laser shots using progressively smaller spot sizes could ablate away a portion of the cornea to effectively build a "corrective lens" into the cornea. These techniques are discussed, for example, in U.S. Pat. No. 4,973,330, entitled "Surgical Apparatus for Modifying the Curvature of the Eye Cornea," issued Nov. 27, 1990, and in U.S. Pat. No. 4,729,372, entitled "Apparatus for Performing Ophthalmic Laser Surgery," issued Mar. 8, 1988. Those skilled in the art of laser ophthalmological surgery have extensively developed the required exposure patterns using these variable size apertures to provide an appropriate amount of correction to various degrees of myopia, hyperopia, and astigmatism, and a combination of these conditions.
These multiple aperture systems, however, suffer a number of drawbacks. They tend to be complicated and inflexible, requiring a number of aperture wheels or masks and only providing standard forms of correction for myopia and hyperopia with circular symmetry and astigmatism with cylindrical symmetry. The human eye, however, tends to have more subtle defects. A system that could accommodate these defects and provide more adaptable solutions, as well as a physically simpler components, would thus be advantageous.
An apparatus for ablating tissue from the eye is shown in U.S. Pat. No. 4,973,330, referenced above. This apparatus includes an excimer laser, the laser beam of which impinges on the cornea, with the axis of the laser beam coinciding with the optical axis of the eye. Furthermore, a field stop limits the area of the laser spot on the cornea illuminated by the laser beam, and the size of this field stop is set in a temporarily variable manner according to the profile of the area to be removed so that the thickness of the area to be removed is a function of the distance from the optical axis of the eye.
The system described in U.S. Pat. No. 4,973,330 permits in this way setting the "laser energy deposited" on the cornea as the function of the distance from the optical axis of the eye, but only under the condition that the distribution of energy (i.e., the power of the laser beam spot) is homogeneous, or at least axially symmetrical. This, however, is a condition that excimer lasers in particular do not always fulfill. Inhomogeneous power distribution results in non-axially symmetrical removal. Moreover, the system described in U.S. Pat. No. 4,973,330 only permits the correction of spherical aberrations, not astigmatism.
An apparatus based on the same fundamental idea is known from U.S. Pat. No. 4,994,058, entitled "Surface Shaping Using Lasers", issued Feb. 19, 1991. That apparatus employs a "destructible field stop mask" instead of a field stop having a temporarily variable aperture.
Another class of apparatus for shaping the cornea by means of removing tissue is known from the various L'Esperance patents. These include U.S. Pat. No. 4,665,913, entitled "Method for Ophthalmological Surgery," issued May 19, 1987; U.S. Pat. No. 4,669,466, entitled "Method and Apparatus for Analysis and Correction of Abnormal Refractive Errors of the Eye," issued Jun. 2, 1987; U.S. Pat. No. 4,718,418, entitled "Apparatus for Ophthalmological Surgery," issued Jan. 12, 1988; U.S. Pat. No. 4,721,379, entitled "Apparatus for Analysis and Correction of Abnormal Refractive Errors of the Eye," issued Jan. 26, 1988; U.S. Pat. No. 4,729,372, entitled "Apparatus for Performing Ophthalmic Laser Surgery," issued Mar. 8, 1988; U.S. Pat. No. 4,732,148, entitled "Method for Performing Ophthalmic Laser Surgery," issued Mar. 22, 1988; U.S. Pat. No. 4,770,172, entitled "Method of Laser-Sculpture of the Optically used Portion of the Cornea," issued Sep. 13, 1988; U.S. Pat. No. 4,773,414, entitled "Method of Laser-Sculpture of the Optically used Portion of the Cornea," issued Sep. 27, 1988; and U.S. Pat. No. 4,798,204, entitled "Method of Laser-Sculpture of the Optically used Portion of the Cornea," issued Jan. 17, 1989. In that apparatus, a laser beam with a small focus spot is moved by a two-dimensional scanning system over the area to be removed. This apparatus, which operates as a "scanner," has the advantage that it can generate any two-dimensional profile of deposited energy "over the area to be removed." Because of the small size of the beam spot, the period of treatment, however, is very great, as power per area unit cannot be raised above a specific "critical" value.
Thus, current techniques do not adequately address the non-linear energy distribution of an excimer laser. The excimer laser includes both large scale and small scale non-linearities in its energy distribution. This can cause over-ablation and under-ablation of certain areas of the eye under treatment. Thus it would be desirable to provide a system that further homogenizes the effective energy deposited on the eye.
Systems that use apertures to create a series of progressively smaller shot sizes also suffer from the disadvantage of creating sharp ridges in the treatment zone of the cornea. Especially near the periphery of the treatment zone, a number of shots are typically required to create the necessary ablation depth at each particular spot size. The typical ablation depth for each shot is 0.2 .mu.m. When multiple shots are required at a single aperture size, the ridge depth reinforces, creating an effective ridge of some multiple of 0.2 .mu.m. For example, five shots would result in a ridge height of 1.0 .mu.m. These sharp ridges in the treatment zone can lead to unwanted epithelial regrowth, especially when correcting high diopter defects. A system that minimizes such ridges would promote smoother epithelial healing, preventing excessive regrowth and allowing the corrected eye to retain its correction for a longer period of time and with more stability.
Before ablating, most current excimer techniques also require physically scraping away the epithelial layer from the eye. This can be a traumatic procedure for the patient, and requires a high degree of precision by the surgeon. Alternative, less invasive methods of removal of the epithelium before ablation of the cornea are thus desirable.
Another problem with current techniques involves "central islands" created during the ablation process. A central island is an area of an ablation profile which is not ablated to a depth proportional to the number of excimer laser shots fired on that particular area. For example, in typical myopia patterns, the greatest depth of ablation is at the center of the pattern. In ablating such patterns, a recurring problem is that the central area is not ablated to as great a depth as is needed to create the proper ablation profile. The causes of this problem are not clear. However, techniques which reduce or eliminate this problem are highly desirable.
Further, present systems typically use either a relatively small spot size of less than 0.50 mm, or variable spot sizes that require the spot size to be adjusted throughout the treatment. A relatively small spot size has serious disadvantages, because it greatly increases the number of shots required to complete a treatment. A variable spot size also has disadvantages, in that it requires complex masking instrumentation to allow the spot size to be adjusted. Reducing or eliminating either of these problems would be greatly desirable.
Another problem that has become apparent is thermal heating. Although an excimer laser is a "cold" laser, which functions by breaking molecular bonds rather than by burning, repeated shots at a particular location will cause the tissue to heat. This limits the maximum shot rate allowed at a particular location. This in turn has historically caused treatments to take at least a certain amount of time, because the maximum shot rate could not be exceeded. Eliminating this limitation would similarly be desirable.